Over the past several years, different control techniques have been employed for the positioning of movable valve members of valve devices utilizing electric motors. Generally, the speed and accuracy at which such valve members can be positioned is of significance. One such application where the speed and accuracy of positioning the movable valve member provides important functional advantages is in the area of electronic throttle control (ETC).
Modern vehicles generally employ some type of electronic throttle control (ETC) system for positioning of the engine intake air throttle valve to achieve the benefits of reduced emissions, increased fuel economy, and improved vehicle drivability. Such systems employ an electronic throttle valve having an electric actuator, such as a brushless DC electric motor, which is coupled to movable throttle plate within the bore of the electronic throttle, thereby forming a butterfly valve for adjusting the amount of air flowing into the engine. Fast and accurate positioning of the throttle plate is required in order to take full advantage of the above described benefits. Additionally, the positioning of the throttle plate in response a desired change of the throttle valve position needs to be aperiodic with minimal transient ripples during settling to avoid excessive system component wear, and increased motor losses.
Most vehicles employing ETC provide a so called limp home mode of operation in the event of an ETC failure. This is typically accomplished by employing opposing springs in the electronic throttle valve for biasing the throttle plate to a predetermined open position, if the electric motor is not energized due to a malfunction. This allows the engine to operate in a high idle condition to permit slow movement of the vehicle with continued operation of the power brakes, power steering, and electrical system. Use of the biasing springs in the electronic throttle valve generally introduces significant nonlinear spring forces, which along with other frictional forces can complicate the positioning of the throttle plate.
In the past, ETC systems have used Proportional-Integral-Derivative (PID) controllers with nonlinear feedback and/or feedforward compensation to account for the frictional forces and nonlinearities of the opposing dual biasing springs (see for example, U.S. Pat. No. 6,523,522, which is assigned to the same assignee as the present invention, and is hereby incorporated by reference). With such ETC systems, the PID gain is usually tuned to provide a motor control signal for the electric motor that achieves the fastest possible end-position to end-position throttle response (closed to open or open to closed throttle plate positions) without saturating the motor control voltage, which is typically bounded by defined voltage limits (typically +12 volts and −12 volts for automotive applications utilizing 12 volt batteries).
As indicated above, the throttle response needs to be aperiodic, without large settling transient ripples when repositioning the throttle plate. This generally requires that the electric motor in an ETC system have a relatively large torque constant and large motor drive currents. Due to these constraints, controllers in ETC systems rarely use the maximum available motor control voltages when generating motor control signals for controlling the electric motors. As a consequence, the response time required for positioning of the throttle valve in such ETC systems tends to be appreciably less than optimal.
Accordingly, there exists a need for a method and apparatus for controlling motor actuated valve devices such as electronic throttle valves, wherein a significantly larger portion of the available actuator control voltage can be utilized when positioning the movable valve members of such valve devices to achieve more optimal control.